Membranes that selectively allow diffusion and adsorption of ions while excluding certain other ions and non-ionized solutes and solvents, typically referred to as ion exchange membranes, have numerous important industrial applications. Such membranes are used in electrodialysis and electrodeionization equipment as well as in devices for fractionation, transport depletion and electro-regeneration, and purification or treatment of water, food, beverages, chemicals and waste streams. The membranes are also used in electrochemical devices such as caustic/chlorine electrolysis equipment, electropaint purification equipment, and electro-organic synthesis equipment. Additionally, ion exchange membranes are used in electrophoresis devices and analytical equipment as adsorbents, and as suppressor devices for ion chromatography. They are used in chemical treatment and concentration applications via the processes of Donnan dialysis and diffusion dialysis, and they are also used in batteries and fuel cells for the production of electricity.
In each of the applications described above, numerous membrane properties must be balanced against one another in order to achieve a membrane that satisfies the desired objectives of the particular application. Among these, it is an objective to employ ion exchange membranes that have high selectivity, low solvent and non-ionized solute transfer, low diffusion resistance of the ions selected, high physical strength, and good chemical resistance. Additionally, it is desirable that such membranes be easily manufactured at low cost without the use of hazardous substances. Furthermore, ideal membranes should be easy to handle and process and should also be amenable to low cost assembly techniques during the production of devices containing such membranes.
Current commercially available ion exchange membranes are primarily of two general types: homogeneous membranes and heterogeneous membranes. A homogeneous membrane is one in which the entire volume of the membrane (excluding any support material that may be used to improve strength) is made from the reactive polymer. Examples include membranes made of sulfonated or aminated styrene-divinylbenzene polymers (SDVB membranes), polymerized perfluorosulfonic acids (PFSO membranes) or various thermoplastics with active groups grafted onto the base polymer.
Unfortunately, homogeneous membranes tend to be difficult to manufacture. They also tend to employ the use of hazardous materials during their manufacturing process since, for the most part, they must be made from base monomers. Additionally, they are difficult to modify chemically because each modification requires a change in the fundamental chemistry of the membrane.
Homogeneous membranes also tend to have limited physical strength (therefore often requiring a screen or cloth support) because the polymer produced cannot readily combine both the required physical and electrochemical properties to operate efficiently in a fabricated device. Homogeneous membranes may be either crosslinked (to provide the membrane with dimensional stability, but increased brittleness and sensitivity upon drying), or they may be non-crosslinked (to provide membranes which may be dried, but lack dimensional stability and resistance to swelling and various solvents).
In contrast, heterogeneous membranes are formed of 1) a composite containing an ion exchange resin to impart electrochemical properties and 2) a binder to impart physical strength and integrity. Typical heterogeneous membranes may be produced as "micro-heterogeneous" membranes by the paste method (in which ion exchange resin monomers are reacted to form the ultimate ion exchange resin polymer in the presence of a finely-ground inert binder polymer), or in the alternative, as "macro-heterogeneous" membranes by the physical blending of pre-polymerized ion exchange resin and binder.
Present macro-heterogeneous membranes tend to have inferior electrochemical properties as compared to homogeneous or micro-heterogeneous membranes, but they do offer a number of advantages as compared to such membranes. In particular, macro-heterogeneous membranes are easy to manufacture and can be readily chemically modified since, within limits, the binder and resin types and content can be varied without significantly modifying the manufacturing process. Macro-heterogeneous membranes also tend to be stronger than homogeneous membranes, although they still generally require a screen or cloth support. Finally, macro-heterogeneous membranes can be dried without damage to the membrane.
Unfortunately, present heterogeneous membranes are still difficult to manufacture. They typically are produced from a solvent-containing lacquer that is dangerous to handle and becomes hazardous waste. Furthermore, such membranes are often limited to the use of a binder that can be dissolved in a relatively non-toxic solvent. Finally, although not damaged upon drying, they do undergo substantial dimensional changes, thus making it difficult to fabricate them into devices in which drying may occur.
The most common macro-heterogeneous membrane is a composite containing a styrene-divinylbenzene (SDVB) based resin, a polyvinylidenefluoride (PVDF) binder, and a polypropylene cloth support. The SDVB is usually mixed into a solution of PVDF dissolved in a solvent such as N-methyl pyrrolidone (NMP) to form a suspension. The suspension is coated onto the polypropylene support, dried in an oven and pressed in a heated press.
The method described above suffers from numerous disadvantages. The PVDF, NMP, and polypropylene cloth are very expensive, as is the manufacture of the suspension and the equipment required for coating, drying and pressing. The suspension itself is moisture sensitive and has a limited shelf life due to settling of the resin. Also, the NMP, after drying and extraction, is a hazardous waste material. Furthermore, although PVDF is reasonably chemically resistant in use, it can be attacked by strong bases and solvents.
The use of the polypropylene cloth, since it is not ionically conductive, has the effect of further restricting the diffusion of ions through the membrane, thereby decreasing the electrochemical properties of the membrane as compared to competitive homogeneous membranes. Also, upon cutting the membrane to a desired size for a particular application, strands from the cloth tend to become exposed, and liquid within the membrane tends to diffuse to the membrane edges through the strands (a problem common to both homogeneous and heterogeneous membranes). This causes a liquid "weeping" or leaking phenomenon that detracts from the appearance and performance of devices such as plate and frame type equipment that have membrane edges exposed to the exterior of the device.
An alternative way to manufacture heterogeneous ion exchange membranes using heat and pressure, as opposed to dissolution coating, described above is also well-known in the art. Such a method is usually used if the binder polymer is not readily dissolvable in a solvent. For example, U.S. Pat. Nos. 2,681,319 and 2,681,320 describe methods for producing heterogeneous membranes and methods for producing a film of controlled thickness. These references also describe post-conditioning of the membrane film using water.
U.S. Pat. No. 3,627,703 extended the heat and pressure technique of forming heterogeneous ion exchange membranes to include polypropylene binders and described numerous film-forming processes including extrusion. The reference describes the use of microscopically foamed and molecularly oriented polypropylene to reduce resin brittleness, thereby overcoming one of the disadvantages associated with such membranes. The reference also notes that if low crosslinked ion exchange resins are used and a hot acid or alkali conditioning procedure is followed, the conditioned membrane is dimensionally stable and pliable even when maintained in an ambient environment. However, the process was found to be ineffective with polyethylene binders, producing a brittle polyethylene membrane.
Subsequently, U.S. Pat. No. 3,876,565 sought to enhance the pliability of the heterogeneous membranes by expanding the ion exchange resins during conditioning. This was done by subjecting the membrane treatment in hot water at greater than 80.degree. C. Further improvements were described in U.S. Pat. No. 4,294,933 which sought to prevent micro-cracks, said to be produced during the formation process described in U.S. Pat. No. 3,876,565, by creating siloxane bridging linkages between the ion exchange resin and a vinyl-silane polyolefin copolymer binder. The reference also describes the use of lubricants in the formulation.
In view of the foregoing, it is clear that a need exists for techniques to allow the manufacture of polyolefin-based heterogeneous ion exchange membranes that are crack-free and pliable and that do not require a support screen to provide strength and integrity. In addition, a need exists for polyolefin based heterogeneous ion exchange membranes that can be formed simply, inexpensively, and without the use or generation of hazardous materials.